The wreckage of a Cessna Citation which crashed on October 13, 2016, is seen in the woods near Lake Country, B.C., in this October 15, 2016, Transportation Safety Board handout image. The aircraft crashed shortly after takeoff, killing the pilot and all three passengers aboard, including the former Alberta Premier Jim Prentice. THE CANADIAN PRESS/HO-TSB,

There is no definitive way to know what caused a small plane carrying former Alberta Premier Jim Prentice and three others to crash into a forested area near Kelowna on Oct. 13 2016, but investigators have constructed a “plausible scenario” hinged on a condition called spatial disorientation.

In a Transportation Safety Board press conference held Thursday morning, investigators explained they constructed a theory based on radar data, the pilot’s flight history as well as the absence of any evidence indicating mechanical failure.

TSB investigator Beverly Harvey said radar data indicates that the plane took off at a high speed appropriate for take-off, and then the speed dropped back dramatically, only to increase again. The plane deviated from its track by 20 degrees and entered a steep descending turn that ultimately ended in the crash that killed all onboard.

What happened is in line with what would happen if a pilot suffered from spatial disorientation.

“(The pilot gets a) feeling of tumbling backwards or vertigo because of the balance system in the inner ear … it can cause a pilot to push forward on controls and to turn thinking that he’s straightening out the aircraft, which is why it is so insidious,” said Kathy Fox, Chair of the TSB.

The condition can cause pilots to doubt the plane’s instruments, doubt its controls, and to put the plane into accidental spiral dive.

The pilot, said Fox, had only seven to nine hours of night flying in the last six months, which she said is not a lot for piloting a high-performance plane. In fact, it falls below industry standards of completing five take-off and landings at night over six months.

Fox stressed that while they believe their theory has merit, pilot error is not a reason they are willing to offer.

There just isn’t enough evidence to say anything conclusively as there was no flight recorder, no voice recorder, no detailed sequence of what happened in flight deck.

There are two things that she did know, however. First, that the plane was not required to have these recording devices onboard and second, that needs to change.

This accident may act as the impetus for change that could affect hundreds of aviation companies and thousands of planes.

“We don’t like having to say ‘We don’t know’ when asked what caused an accident and why,” said Fox, Chair of the TSB. “We want to be able to provide definitive answers—to the victims’ families, to Canada’s aviation industry, to the Canadian public. This is why we are calling today for the mandatory installation of lightweight flight recording systems on commercial and private business aircraft not currently required to carry them.”

The Board also raised a concern with the way Transport Canada had conducted oversight of private business aviation in Canada.

During the course of its investigation, the TSB found no record that the operator of this aircraft had ever been inspected by TC. As such, TC was unaware of safety deficiencies in its flight operations, such as the failure to obtain approval for single-pilot operation of the aircraft and the pilot’s lack of recent night flying experience required to carry passengers at night.

Since this occurrence, TC has said that it will conduct targeted inspections of private business operators starting in April 2018. Fox said the TSB will continue to monitor this safety issue.

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UPDATE: 7 a.m.

The investigation into the 2016 plane crash that killed four passengers, including former Alberta premier Jim Prentice, has been completed and results will be announced this morning.

The Transportation Safety Board will hold a news conference at 9 a.m. to release its investigation report on the Oct. 13 2016 crash of the Cessna Citation 500 just outside Kelowna, and the Capital News will report on its findings.

At the time of the crash RCMP Cpl. Dan Moskaluk said the Cessna Citation private business aircraft was owned by Norjet, a Calgary-based firm, and had departed Kelowna International Airport en route to Springbank, outside of Calgary.

“I know that heavy rain was felt across the Okanagan Valley but difficult to say if that was an issue in this case,” Moskaluk said.

The Transportation Safety Board released a statement at the time also, saying that there were no emergency calls or signals from the high impact crash that scattered debris over a vast space.

Kelowna RCMP and Lake Country RCMP detachments were alerted by the Surrey Air Traffic Control Centre that they had lost contact with a Citation jet shortly after its takeoff from Kelowna airport.

Also onboard that night were Sheldon Reid, a shareholder in Norjet, the owner of the Cessna Citation aircraft that crashed, and Jim Kruk, associated with the 83 ‘Lynx’ air cadet squadron based out of Airdrie, a rural community north of Calgary, and Calgary optometrist Kenneth Gellatly, whose son was married to Prentice’s daughter.

The men were flying home to Calgary from a late-season golf game in Kelowna when the crash occurred.